1. Field of the Invention
The present invention relates to a static excitation system that can eliminate shaft vibrations of a generator and overvoltage when it is underexited. More specifically, the present invention relates to a static excitation system that can eliminate shaft vibrations of a generator and transient phenomena generated in a prior art, so that the present invention can be applied in a field in which the excitation system has a great importance in an isolated system or a weak system.
2. Description of the Related Art
According to prior arts, generators having a small capacity have been used to produce excited voltage, since techniques for generating high power had not been developed enough. However, as development techniques for a semiconductor device are advanced, a thyristor is used to receive power from an output terminal of a generator and supply excited power of the generator instead of the generator having a small capacity as shown in FIG. 1.
Since the generator having a small capacity is not used in the aforesaid system embodying the thysristor, it has some advantages, such as quick response and broad control area. However, when voltage of an output terminal of a generator has dropped, power supply of the excitation system is also dropped, and when output of the generator is increased by regulating the phase of the thyristor provided in the excitation system, output of the generator is increased again, thereby these steps are repeated again, i.e., output of the generator is dropped again by regulating the phase of the thyristor, and thereby vibrations of the output voltage of the generator could be arisen by the repeated steps.
At this time, nonlinear characteristics of a current limiter or the system itself have been contained in operating features of an excitation controller. When the vibrations of output voltage of a generator coincide with mechanical resonance frequencies of a generator-turbine system, vibrations of output voltage of a generator are changed to resonance, wherein the resonance means that output voltage of a generator is gradually increased. Finally, since the resonance may incur a damage on a shaft of the generator-turbine, it is needed to prevent that the output voltage of the generator influences control characteristics of the excitation system.
FIG. 1 is a diagram illustrating voltage control system of a generator using a conventional thyristor. FIG. 1a is a diagram illustrating a static excitation system using the conventional thyristor, and FIG. 1b shows a construction of a rotational excitation system using the thyristor in which direct current is supplied to a rotor (field magnet) using a thyristor converter 12 from an output terminal of a generator 14 via a step-down transformer 11. There are 6 thyristors A-C and Axe2x80x2-Cxe2x80x2 in the converter 12, which are controlled by gate signals of a controller 13.
According to FIG. 1b, AC power is generated by PMG (permanent magnetic generator; 15) and transformed into DC power via a thyristor converter 16. Direct current is then supplied to a field magnet of an auxiliary generator (excitation type generator; 18) to control the voltage of the auxiliary generator 18. Output of the auxiliary generator 18 supplies direct current to a field magnet of a main generator 14 via a diode bridge 17 to control the voltage of the main generator 15.
In case that excitating current is dropped to drop down the voltage of a generator, since the thyristor type excitation system shown in FIG. 1, owing to the characteristics of the thyristor, can flow current in just one direction, overvoltage is generated by discriminator current, and the overvoltage make a damage on the excitation system. If the exiting current also can flow in a contrary direction, the overvoltage could be eliminated.
As shown in the FIG. 1a, in the static excitation system using the conventional thyristor, an ignition angle of a thyristor constituting a thyristor bridge after excitation power is inputted from the output terminal of the generator 14 to control exciting current which is supplied to a field magnet of a synchronous generator. Thereby the terminal voltage of the synchronous generator is maintained in constant.
The static excitation system has some advantages that it is convenient to construct and also easy to maintain. However, The properties of the excitation system are directly affected by the changes of the output voltage of the synchronous generator because that excitation AC power is supplied from the terminal of the synchronous generator. One of the problems which may be caused by that the output terminal of the generator is directly connected to the excitation system, is that input of the excitation system is decreased according to reduction of the terminal voltage of the generator and therefore, the excitation system should conduct an operation for increasing the output of the generator in order to enhance the output of the generator.
This operation results resonance (vibrations) of the output voltage of the generator. The resonance (vibrations) is more frequently happened in a system having properties of high gain or quick response. If the resonance corresponds with mechanical resonance frequency of the generator, a shaft of a generator-turbine system could be broken. Accordingly, It is need to implement a controller which is designed considering the shaft vibrations of the generator-turbine system and a PSS (power system stabilizer) eliminating the frequencies of the shaft vibrations of the generator-turbine system in the excitation system. However it is difficult to implement the aforesaid strategy, and hard to satisfy all requirements because the frequencies of the shaft vibrations are not one fixed value but having a several different values.
Since the excitation system using a thyristor can flow current in one direction, in a system in which a condenser bank is provided or a system having Ferranti""s effect, i.e., voltage of a receiving end becomes higher than that of a sending end, continuous reducing of the terminal voltage may cause that the voltage applied on a rotor of the generator becomes to be discontinuous or the current becomes zero. At this time, since the rotor of the generator has a high inductance, overvoltage (V=L; di/dt) is applied on the both terminals of the thyristor by the discontinuous current instantly so that it makes a damage on the excitation system of the generator. In order to prevent this overvoltage, it contains a reverse resistor or a crowbar circuit. However, since degree and duration of the overvoltage is differed according to the properties of the system, it is hard to adjust. Otherwise, if the current flows in both directions not only one direction, the overvoltage is disappeared in the system and pole slipping can be prevented or delayed.
The present invention intends to improve disadvantages of a conventional excitation systems using GTO (gate turn off thyristor) or IGBT (insulator gate bipolar transistor), or a conventional thyristor type excitation systems, i.e., preventing shaft vibrations of a generator which can be generated in a static excitation system having high gain property and overvoltage which may produced in a system having one directional current continuity.
The present invention is a novel static excitation system of a synchronous generator controlling terminal voltage of the synchronous generator in a power plant. The present invention solves the problem related with shaft vibrations which may produced in a static excitation system having a properties of quick response and high gain in order to supply exciting current rapidly, which is needed to a generator when an output terminal of the generator shows an extraordinary phenomena or gets out of order. Furthermore, the present invention also solves the problem related with overvoltage of a system which may produced in a low excitation condition in a long distance line causing Ferranti""s effect or in a case that it is connected to HVDC (high voltage direct current).
The purpose of the invention is accomplished by an excitation system comprising:
an initial excitation equipments;
a step-down transformer;
a 3-phase diode bridge;
a boost chopper provided with a transistor and a resister, and maintaining DC voltage to be constant;
a DC chopper provided with a plurality of transistors and diodes, and supplying DC power to a rotor of a generator;
a boost controller for controlling the boost chopper; and
a DC chopper controller for controlling the DC chopper,
wherein, overvoltage applied on the excitation system when underexcited is eliminated owing to the boost chopper preventing any changes at an output terminal of a generator from being transferred to the excitation system, and maintaining the excitation DC voltage to be constant; and also owing to the DC chopper which is capable of 4 quadrant operation, thereby pole slipping of a generator can be prevented.